Signal generators take many forms, from (a) a basic analog oscillator circuit that oscillates at a frequency governed by an RC (resistor capacitor) network, or the resonance of an LC (inductor-capacitor) circuit or the resonance or delay of devices such a SAW (surface acoustic wave) device, or a crystal, or a dielectric resonator, or BAW (bulk acoustic wave) device, etc., to (b) an analog oscillator followed by a non-linear stage that generates a harmonic term that is isolated by a filter to serve as the output tone, to (c) a combination of analog and digital circuits that may lock a higher frequency oscillator with poor long term stability but good short term stability, to a low frequency oscillator with good long term stability, but poor short term stability, such as phase-locked-loop (PLL), to (d) an NCO (numerically controlled oscillator) that is formed by driving a DAC (digital to analog converter) with data that causes the DAC to produce the desired output signal. Each method results in different sets of advantages and disadvantages in terms of metrics such as size, cost, weight, power consumption, settling time after a frequency change, modulation bandwidth, modulation linearity, long and short term stability, harmonic levels, spurious levels, and flexibility or ease of control.
Numerically controlled oscillators are well known to achieve optimally fast (short) settling times. At the same time, due to a combination of their discrete-time digitally-sampled stair-step output waveform, and the fact that the stair-step output levels must settle in each time-step period, an NCO cannot be clocked fast enough to produce the bandwidth required by many applications. It is an objective of this disclosure to show a method of obtaining arbitrarily large bandwidths.
Many applications are extremely sensitive to spurious signals. Applications such as radar, ladar (LAser Detection And Ranging), sonar, and numerous instrumentation applications, are fundamentally limited by the SFDR of the signals they generate and use to perform their function. The limit follows from the fact that they rely on correlations on these signals. Harmonic and spurious signals correlate along with the desired signal, causing an error in the measured correlation, which is supposed to only come from the desired signal.
One method of extending the bandwidth of an NCO is to use a frequency multiplier. This can be done using common mixer circuits or by using a circuit that generates harmonics and then filtering out all the harmonics that are undesired. This method results in a high noise floor and is unsuitable for many applications. Additionally, if the initial waveform was a complex waveform with, for example, a desired spectral notch, the multiplication process would destroy the notch. Another objective of this disclosure is to extend the bandwidth yet maintain a low noise floor, and preserve characteristics like spectral notches in the initial waveform.
Another method of extending the bandwidth of an NCO is to use a mixer together with a large N-way switch network and family of N tones spaced in frequency by the bandwidth (B) of the NCO. The switch network selects the desired tone and the NCO output signal is shifted in frequency by the frequency of the selected tone. In this way, the bandwidth covered by the output signal is N times the bandwidth of the NCO. This method results in a high power and a physically large and costly system that is incompatible with many applications. Another objective of this disclosure is to extend the bandwidth, yet at low power, a small footprint, and at relatively low cost.
Another method of extending the bandwidth of an NCO is a variant of the above paragraph, where the set of N oscillators is replaced by one or a smaller number of PLL synthesizers which are capable of being commanded to generate the N needed frequencies. The relatively slow switching speed of the PLL's in this solution, however, makes it unsuitable for many applications. It also prevents the system from continuously sweeping across the extended bandwidth since the PLL settling time is so long. Another objective of this disclosure is to extend the bandwidth at low power and small footprint, and with the capability to sweep the entire extended bandwidth continuously, and with optimally fast hopping or switching speed.